This invention relates to a noninvasive method of measuring temperature by using ultrasound and more particularly to a method of determining local tissue temperature in the application of hyperthermia techniques by measuring the velocity of ultrasound in the region of interest.
It is important in the application of hyperthermia techniques using ultrasonic or microwave applicators to monitor the temperature of internal tissue structures as they are being treated. Although interactions of ultrasound with tissue are mostly insensitive to temperature, it has been found that the velocity of sound in various tissue structures is dependent upon temperature and attempts have been made to develop techniques for measuring the average local velocity of sound in a defined spatial volume of tissue in order to determine the temperature and/or other physical characteristics of the tissue from the results of such measurement.
Such techniques have typically involved the use of computed tomography principles for the measurement of the local velocity within small elements throughout the entire cross sectional region of the anatomy of interest. By such techniques, the transit time required to traverse the plane region of interest is measured in all directions. This may be done, for example, in a parallel or fan beam geometry as in the case of standard X-ray CT systems. These resulting transit times correspond to the line integrals for X-ray attenuation and in principle can be substituted into a reconstruction algorithm for yielding results corresponding to the local ultrasound velocity within small elements throughout the plane region which has been measured. An important problem with this technique is the effects of refractions which cause the rays to travel along a curved path, thereby making image reconstruction difficult, if not impossible.